Generally, a storage apparatus has a cache mechanism mounted thereon to improve performance thereof. The cache mechanism is a mechanism which arranges (caches) data which is frequently used or data which has temporal locality, and hides a high latency of reading and writing from and in a HDD (Hard Disk Drive). This cache mechanism uses characteristics that an access latency from a CPU (Central Processing Unit) to a memory is remarkably low compared to reading and writing from and in the HDD.
Recently, in a storage field, big data is actively researched and developed as a keyword. A total capacity of a big data storage reaches several tens to several hundreds of PBs (petabyte) and is said to reach 1 EB (Exabyte) in near future.
TOC (Total Cost of Ownership) is becoming a problem. When, for example, a SAS (Serial Attached SCSI (Small Computer System Interface)) drive of 1 TB (terabyte) is used to incorporate a system having a total capacity of 1 EB, 1,000,000 drives are required and electricity cost goes up to several hundred thousand dollars per month.
It is indispensable to power off power sources of the drives to reduce this enormous electricity cost. However, even though the power sources of the drives are powered off, it is necessary to power on the drives when data is read and written from and in the drives. Hence, there are some cases where a drive is carelessly activated in response to a user's request and, as a result, reduction in electricity cannot be achieved.
A method called write off-loading is known as a method of reducing a power consumption amount of such a storage apparatus.
According to the write off-loading, when data is written in a drive which is powered off, data is written (offloaded) to a data storage area (log area) which is not used in another drive which is powered on. Further, when the original writing destination drive which is powered off is powered on, the offloaded data is written in (written back to) this drive.
Consequently, it is not necessary to unnecessarily awake the drive even when writing data in a drive which is powered off is requested, and it is possible to reduce power consumption.
However, in such a conventional storage system, when offloaded data is written back, the written-back data is no longer used in the write-back source drive and does not deserve being cached.
That is, immediately after data is written back in the write-back source drive, worthless data having an offloaded data size is stored in a cache. Therefore, there is a problem that the conventional storage system cannot efficiently use a cache area after offload data is written back.